A COMPARISON OF CATALYTIC ACTIVITY FOR IMINE HYDROGENATION USING RU DITERTIARY PHOSPHINE COMPLEXES, INCLUDING CHIRAL SYSTEMS

A family of ruthenium ditertiary phosphine complexes was investigated for catalytic activity toward imine hydrogenation. The diphosphines (PP) used include chiral (chiraphos, diop, binap) and achiral (dppe, dppb) systems (chiraphos = Ph2PCH(Me)CH(Me)PPh2; diop = Ph2PCH2CHOCMe2OCHCH2PPh2; binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl; Ph2P(CH2)(n)PPh2 (n = 2, dppe; n = 4, dppb)). Activity was observed in MeOH at low catalyst concentrations (0.77 mM Ru), under moderate conditions (room temperature (r.t.), 1000 psi H2). The air-stable Ru2Cl5(PP)2 complexes were more active than the commonly used dimeric Ru2(II,II) systems to which they give rise in situ. Asymmetric induction in the prochiral ketimine PhCH2N = C(Me)Ph was consistent, though modest, within the diop and binap series of neutral complexes, implying a common catalytic intermediate within each series. A maximum e.e. of 27% was found using Ru2Cl5(chiraphos)2 as catalyst; this represents a non-optimized figure for asymmetric induction, as no other prochiral ketimines were screened.